1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine and a printer using an electrophotographic technique, and a process cartridge mountable to such an image forming apparatus, and more particularly, it relates to an image forming apparatus and a process cartridge, in which density of an image formed on a photosensitive member can automatically be adjusted.
2. Related Background Art
FIG. 3 is a sectional view of an image forming apparatus having an image density adjusting function. In this apparatus, a photosensitive drum (electrophotographic photosensitive member) 1, a charge roller (charge means) 2 and a cleaning means 7 are integrally provided as a process cartridge 100, which is constructed as shown and is detachably mounted to a frame 90 of the image forming apparatus via a mounting guide means 80.
While the photosensitive drum 1 is being rotated in a direction shown by the arrow by a drive means (not shown), a surface of the photosensitive drum is uniformly charged by the first charger 2 with predetermined potential. Then, laser light corresponding a magenta image pattern is illuminated on the photosensitive drum 1 through an exposure apparatus 3 to form an electrostatic latent image on the photosensitive drum 1.
As shown in FIG. 5, the photosensitive drum 1 is constituted by a conductive (such as aluminium) drum base 1a and a photosensitive layer 1b made of organic photo-conductor (OPC) and coated on the drum base, and the drum base 1a is grounded. The photosensitive layer 1b has four-layer structure including a conductive layer (CL) 1b-1, an injection preventing layer (IPL) 1b-2, a charge generating layer (CGL) 1b-3, and a charge transporting layer (CTL) 1b-4.
Four developing devices 4 (4a, 4b, 4c and 4d) are mounted on a support 5 at a downstream side of the process cartridge 100 in a rotational direction of the photosensitive drum 1. Each of the developing devices 4a to 4d is formed as a cartridge which can be detachably mounted on the support 5 (and, accordingly, on the body 90 of the image forming apparatus). When the photosensitive drum 1 is rotated in the direction shown by the arrow, the support 5 is rotated so that the developing device 4a including magenta (M) toner among four developing devices is opposed to the photosensitive drum 1. The latent image formed on the photosensitive drum 1 is developed by the developing device 4a so selected, to thereby visualize as a magenta toner image. The magenta toner image is transferred onto an intermediate transfer belt (intermediate toner member) 5. While the intermediate transfer belt 5 is being rotated in a direction shown by the arrow at substantially the same speed as the photosensitive drum 1, by applying primary (first) transfer bias to a first transfer roller 14 opposed to the photosensitive drum 1 with the interposition of the intermediate transfer belt 5, the magenta toner image formed on the photosensitive drum 1 is primary-charged on an outer peripheral surface of the intermediate transfer belt 5.
By repeating the above-mentioned processes successively regarding a cyan color (C), a yellow color (Y) and a black color (K), a color image is formed on the intermediate transfer belt 5 by superimposing the magenta, cyan, yellow and black toner images. A transfer material (such as a paper sheet) is picked up from a transfer material cassette 11 by a pick-up roller 12 at a predetermined timing, and the picked-up transfer material is supplied to the intermediate transfer belt 5, and, at the same time, by applying second transfer bias to a second transfer roller 8, the four color toner images on the intermediate transfer belt 5 are collectively transferred onto the transfer material. The transfer material to which the four color toner images were transferred is sent to a fixing device 6, where the toner images are fused and mixed by heat and pressure, to thereby form a full-color image. Residual toner remaining on the photosensitive drum 1 is cleaned by a conventional cleaning means 7 including a cleaning blade.
Within the body 90 of the image forming apparatus, a density sensor 9 is disposed in the vicinity of the photosensitive drum 1. In general, in color image forming apparatuses of electrophotographic type, when image density is changed in accordance with various conditions such as an environmental condition (temperature, humidity), the number of copy sheets and the like, the correct color of the color image itself could not be obtained. Thus, density detecting test images (patch images) are formed on the photosensitive drum 1 with various color toners, and density of each image is detected by the density sensor 9. On the basis of the detection results, image density control is effected through feed-back of detection results of the exposure amount and the developing bias, to thereby obtain a stable image.
As shown in FIG. 6, the density sensor 9 comprises a light emitting element 91 such as LED, a light receiving element 92 such as a photo-diode, and a holder 93. In this case, an infrared ray from the light emitting element 91 is illuminated on the patch image P on the photosensitive drum 1, and light reflected from the patch image is received by the light receiving element. By measuring an amount of the reflected light, density of the patch image is detected. The light reflected from the patch image includes specular reflection components and irregular reflection components. Since a light amount of the specular reflection components is greatly changed in accordance with a condition of the surface of the photosensitive member as a background for the patch image and/or a distance between the sensor 9 and the patch image P, if the specular reflection components are included in the light reflected from the patch image P, the detection accuracy will be greatly worsened. To avoid this, in the density sensor 9, an angle of illumination light incident to the patch image P is selected to 45.degree. and an angle of the reflected light (reflected from the patch image P) received by the light receiving element is selected to 0.degree. so that the specular reflection components from the patch image P do not enter into the light receiving element 92, to thereby measure the irregular reflection components alone.
Further, when the light emitting amount is decreased by degradation of the LED as the light emitting element 90 or if a measuring surface of the sensor is smudged by toner, it is difficult to maintain the initial ability of the density sensor 9 as it is. To cope with this, there has been proposed a method for correcting of the density sensor 9 by adjusting drive current of the light emitting element 91 so that an output (light receiving amount) of the light receiving element 92 shows a predetermined value (see Japanese Patent Application Laid-Open No. 7-36230.
However, this density sensor correcting method is based on the assumption that the colors on the photosensitive drum, i.e., reflection factors have no dispersion independently. In order to effect the correction of the density sensor 9, it is required that all of photosensitive drum are manufactured to have no dispersion in their reflection factors, to thereby increase the manufacturing cost of the photosensitive drum. Particularly, in the image forming apparatus of process cartridge type as shown in FIG. 3, since the cartridge must be exchanged, all of the photosensitive drums in the respective cartridge must have the same reflection factor.
Further, upon the so-called image density control in which, prior to normal image formation, density detecting image (patch images) are formed on a photosensitive drum to detect, and density of each image by density sensor comprised of a light emitting element and a light receiving element, and, on the basis of the deflection results, various image forming conditions such as charge potential of the photosensitive drum, a light amount of an exposure device and developing bias to be applied to a developing means are controlled, when a cartridge (process cartridge or developing cartridge) is exchanged, the image density is changed due to dispersion in sensitivities of the photosensitive drums and/or dispersion in frictional charging features of toners.
Although various efforts for stabilizing such variable factors have been made, satisfactory result has not yet been achieved. Thus, particularly, in the color image forming apparatuses, in order to obtain desired density and color balance, four color (Y, M, C and K) image forming conditions must be adjusted, and, hence, it is advantageous that the image density is automatically controlled by making the above control automatic. Further, the above control may be performed when the power source of the image forming apparatus is turned ON, when each cartridge is exchanged and when a predetermined number of copies are finished. In particular, when a temperature/humidity sensor for detecting temperature and humidity in the image forming apparatus is provided, such control may be performed only if predetermined change in temperature/humidity occurs.
However, in the above conventional image forming apparatus, since the image forming conditions must be controlled in consideration of all variable factors, a number of patch images having different image forming conditions must be formed, and, thus, the control time is increased accordingly and a large number of toner is consumed. To avoid that, it is considered that a smaller number of patch images are formed. In this case, however, control error is increased accordingly, which causes poor color balance particularly in the color image. Alternatively, upon manufacturing, only photosensitive drums and toners having no or less dispersion may be selected. In this case, however, yield is greatly worsened, to thereby make the photosensitive drum and toner expensive.